PRODUCTION OF TELEVISION SIGNALS 



563 



the apertures trace across the Image one after the other in a series of 

 parallel lines. The frame limits the size of the image and prevents 

 more than one aperture being in the image at one time. Light, 

 passing through an aperture as it travels across the image, falls in 

 the light-sensitive cell and generates a picture current proportional 

 to the brightness of the Image from point to point along strips taken 

 one after the other across the image. 



Fig. 3 — Illustrative transmitting apparatus. Light from the arc lamp is con- 

 densed on the disk, which is driven by a high frequency synchronous motor. The 

 disk carries a spiral of pin hole apertures, each of which in turn projects a moving 

 spot of light on the subject. Reflected light is collected by three large photo- 

 electric cells. 



In any system such as that outlined above, which depends upon 

 scanning an Image of the view as formed by a lens, the efficiency of 

 the system is ultimately limited, for any given size of image that 

 can be scanned, by the ratio of aperture to focal length of the best lens 

 that can be secured. Experiments show that, with the best lens 

 available to form a one-inch-square image, it would be necessary to 

 Illuminate a subject with a 16,000-candle power arc at a distance of 

 about four feet In order to secure an image bright enough for a photo- 

 electric cell to give an output current above the noise level in an 

 amplifier system. In other words, television would apparently be 

 extremely Inconvenient to the subject if it were to be carried out from 

 an Image formed by a lens. 



